Bleaching of hardwood sulfite pulp with hydrogen peroxide, including pretreatment with alkali



United States Patent 3,382. 149 BLEACHTNG 0F HARDWOOE) SULFITE PULP WETH HYDRQGEN PERGXHDE, HNCLUDING PRETREATMENT WliTH ALKALI George L. K. Huh, Wilmington, DeL, assignor to E. ll. du Pont de Nemours and Company, Wilmington, Deh, a corporation of Delaware No Drawing. i iied Oct. 29, 1964, Ser. No. 407,558 6 Claims. (Cl. 162-78) This invention re.ates to the bleaching of high yield hardwood sulfite pulps.

The common sulfite pulps are generally produced by cooking softwoods, e.g., spruce, with an acidic, e.g, pH 2.53.5, sulfite cooking liquor for several hours under steam pressure at temperatures corresponding to the pressure employed. Such pulping procedures result in a relatively low-yield (about 50%) pulp from which materials such as lignin have been largely removed. Such pulps are generally bleached by multi-stage operations which in elude a chlorination stage to solubilize residual lignin which is removed by extraction.

In contrast to such common low-yield sulfite pulps are in the high-yield sulfite hardwood pulps derived from poplar, oak, elm, gum, beech, birch, maple, ash or other hardwoods or mixtures thereof. There pulps are produced by subjecting hardwood chips to a mild suifite cook, generally for only a few minutes, e.g. 1 to minutes, to soften the chips, following which the chips are fiberized or disintegrated in a pulp refiner. The sulfite cook may be carried out under alkaline, neutral or slightly acidic conditions, the pH generally ranging from about 5 to 12. The purpose and effect of the sulfite cook are primarily to soften the chips sufficiently to permit effective fiberization in the refiner. The resulting pulp is a high-lignin pulp obtained in yields of from 70-95% and more commonly from 80 to 95%, based upon the weight of the initial wood. These high-yield hardwood pulps are highly useful in supplementing or replacing softwood groundwood or as fillers for chemical pulps in the paper furnish. They yield papers of higher strengths than do groundwoods and they are less expensive than regular sulfite pulps.

These high yield hardwood sulfite pulps vary in brightness from about to 70% as read on a Hunter Multipurpose Reflectometer calibrated against a General Electric Brightness Tester. Such brightnesses often require that the pulps be bleached for most purposes. Since their economic advantage over the more common low-yield sulfite pulps is their high-yield resulting from their high lignin content, bleaching must be achieved without substantial reduction in their lignin content, otherwise such economic advantage is dissipated. Thus, the use of bleaching agents which remove lignin is generally precluded. Hydrogen peroxide and metal hydrosulfites are the only present commercial bleaching agents which do not remove lignin. However, metal hydrosulfites, although quite effective in single-stage bleaching of groundwoods, are surprisingly quite ineffective in single-stage bleaching of these high-yield hardwood sulfite pulps. The reason for this is not understood but the indications are that the two types of pulps are quite different in some fundamental respect. Single-stage bleaching of these pulps with hydrogen peroxide is also not as effective as is desired.

It is an object of the invention to provide an improved method of bleaching such high-yield hardwood sulfite pulps. A particular object is to provide an improved method of bleaching such pulps with hydrogen peroxide, which method involves subjecting the pulp to a pretreatment with an alkali prior to bleaching the same with peroxide, whereby the effectiveness of the peroxide bleach is substantially increased. Still further objects will be apparent from the following description.

The objects of the invention are achieved by subjecting a high-yield hardwood sulfite pulp to the action of an aqueous solution of an alkali under the specific conditions set forth below, following which the pulp is washed then bleached with an alkaline peroxide solution. Although not essential, the peroxide bleached pulp is advantageously bleached further with a metal hydrosulfite such as sodium or Zinc hydrosulfite when an especially high brightness is desired.

It has been found that by pretreating the pulp with a solution of an alkali in accordance with the invention, the pulp can be bleached with alkaline peroxide much more efficiently than when such pretreatment is omitted. Brightness increases up to as much as 6 and more percentage points can be gained as result of such pretreatment. However, to be effective it is essential that the pretreatment be carried out under certain time and temperature conditions so as to facilitate subsequent bleaching. it is essential also that the alkali content of the solution used in the pretreatment be within specified limits. When carrying out the alkali pretreatment and the bleaching with peroxide as described herein, significantly improved bleaching can be realized with no significant decrease in pulp yield.

The alkali pretreatment is preferably effected at a pulp consistency of 5 to employing a treating solution containing 0.5 to 1.5% caustic soda, based upon the dry weight of the pulp, at a temperature of 20 to C. for a time of 0.5 to 1.5 hours, following which the pulp is washed and then bleached with alkaline peroxide. The alkali applied to the pulp, on a dry weight basis, should generally be at least 0.3% but not higher than 3% since contents lower than 0.3% or higher than 3% do not usualiy result in sufficient enhancement of bleaching to be worthwhile. In general, the higher the caustic soda content within the range 0.3 to 3.0%, the lower and/or the shorter should be the temperature and time, respec tively of the pretreatment. At alkali contents less than 2% caustic soda, temperature and time do not appear to be particularly critical, however, the time should generally be at least 0.1 hour and times longer than about 2 hours generally result in no added advantage. At alkali contents greater than about 2%, time and temperature conditions become more critical and when the temperature is as high as C., times longer than about 0.5 hour are distinctly less effective than are shorter times. Regardless of the caustic soda content, temperatures in excess of C. and times in excess of 3 hours are generally best avoided. Accordingly, the pretreatment of the invention should be carried out at a temperature not exceeding 70 C., e.g., 15 to 70 C., for a time which, depending upon the temperature and the amount of caustic soda employed, will result in a substantial enhancement of the bleaching subsequently effected with alkaline peroxide.

Alkalies other than caustic soda can be employed as the active agent of the pretreating solution. Such alkalies include potassium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate and trisodium phosphate. When used, such other alkalies should be employed in amounts equivalent to 0.3 to 3.0% NaOH, which amount should result in an initial pulp-treating solution mixture having a pH of about 10 to 12. The preferred alkali is caustic soda.

Following the alkali pretreatment, the pulp is Washed and then bleached with an alkaline peroxide solution. Alkaline peroxide solutions and bleaching conditions known to be effective for bleaching either groundwood or sulfite pulps can be used. Generally, the bleaching will be effected at pulp consistencies of 10-20%, however,

higher consistencies up to 40% can be used, particularly when long bleaching times at about room temperature are employed. At bleaching temperatures of 40 to 75 C., particularly at medium pulp consistencies, times of 1 to 4 hours are generally sufficient. The bleach solution should be alkaline and generally will have an alkalinity such that the pulp mass will have a pH of between about 10 and 12. The solution preferably will contain between 3 and 7.5% sodium silicate, based upon the dry weight of the pulp, and will have a total alkali content calculated as NaOH (including NaOH equivalent to the Na O content of the sodium silicate), equal to 0.5 to 4.0%, based upon the dry Weight of the pulp. The bleach solution may also include heavy metal sequestering agents such as the alkali metal phosphates and polyphosphates and diethylene triamine pentaacetic acid (DTPA), or its soluble salts or similar aminopolycarboxylates and peroxide stabilizers such as magnesium sulfate. The peroxide content will depend mainly upon the degree of bleaching desired but generally will range from 0.1 to 2.5% H based upon the pulp weight.

The invention is illustrated by the following examples in which all reagent amounts are expressed as weight percentages based upon the dry weight of the pulp unless stated to be otherwise. All brightness values are percent brightnesses read with a Hunter Multipurpose Reflectometer which had been calibrated against a General Electric Brightness Tester.

EXAMPLE 1 The pulp used in the following experiments was a commercial 93% yield poplar sulfite pulp produced by cooking poplar chips briefly with a sulfite liquor having a pH of about 10.5 and then fiberizing the softened chips in a refiner. The pulp samples used in Experiments a and b were acidified with sulfuric acid to a pH of 3.5 at about 1% consistency, then thickened and washed twice with water. The purpose of the acid treatment was to remove heavy metal contaminants which would catalyze decomposition of the peroxide in the bleaching stage. The brightnesses of the pulp before and after the acid treatment, respectively, were 53.7% and 54.7%. The pulp sample used in Experiment 0 was not acid-treated.

(a) A sample of the acid-treated pulp was bleached for 1 hour at a pulp consistency of 15% at 71 C. with a bleach liquor containing 1.25% H 0 (100% basis), 5.0% sodium silicate solution (41.6% Re, 8.9% Na O, 29% SiO 0.05% magnesium sulfate and 2.35% NaOH. After the 1 hour bleaching time, only 1.3% of the original peroxide remained and the bleached pulp had a brightness of 72.6%.

For comparison, a single stage bleaching of the same pulp with 1% sodium hydrosulfite for 1 hour at 60 C., a pulp consistency of 4% and a pH of 6 gave a pulp with a brightness of only 63.1%. This response to bleaching with hydrosulfite is much poorer than the responses of the usual groundwoods to similar bleaching.

(b) A second sample of the acid-treated pulp was treated for 0.5 hour with 1% caustic soda at 15% consistency and 43 C. The pulp was then washed twice, neutralized to pH 6 with dilute sulfuric acid, then washed twice more, at which point the brightness of the pulp was 55.8%, compared with a value of 54.7% before the alkali treatment. The pulp was then bleached with alkaline peroxide as described in (a) above, except the bleaching time was extended. After 3 hours bleaching time, the peroxide remaining was 16.5% of the original and the brightness was 78.6%. A comparison of this value with the brightness of the bleached pulp in (a) above shows that an increase of 6 brightness points was attributable to the caustic soda treatment which preceded bleaching.

(c) A sample of a pulp which was the same as that used in (a) and (b) above except that it had not been acid-treated, was treated with caustic soda and then Y 4.- bleached with peroxide as described in (b) above. The brightness of the bleached pulp was 74.0%.

The following summarizes the results of the above experiments:

A sample of a commercial about 90% yield poplar sulfite pulp made from arsenate killed wood was treated with acid as described in Example 1, then treated with alkali and bleached with peroxide as described in 1 (b). It was then given a supplemental bleach with 1% sodium hydrosulfite in a single stage for 1 hour at 60 C., 4% pulp consistency and a pH of about 6. The brightness after the peroxide bleach stage was 70.8% and after the final hydrosulfite bleach stage it was 72.8%. The brightnesses of this pulp before and after the acid treatment, respectively, were 45.3% and 51.4%.

A repeat experiment was carried out under the same conditions except that the alkali treatment preceding the peroxide stage was omitted. In this instance, the brightnesses following the peroxide stage and the hydrosulfite stages were 66.6% and 70.6%, respectively. Thus, the use of an alkali treatment followed only by a peroxide bleach gave as a bright a pulp as did the peroxide-hydrosulfite bleaching sequence in the absence of the alkali pretreatment. Although the pulp used in these experiments was somewhat more resistant to bleaching than that used in Example 1, the results obtained clearly demonstrate the usefulness of the alkali pretreatment.

EXAMPLE 3 Samples of the acid-treated pulp of Example 1 were subjected to alkali treatments under varying conditions of alkali concentration, temperature and time, then bleached as described in 1 (b). The bleaching results are tabulated below.

Alkali Pretreatment Percent Bright- NaOH-Cone., Temp, C. Time, hrs. ness After percent Bleaching No alkali pretreatment 72.6 0. 5 24 0. 5 74. 2 0. 5 2A 1. 0 74. 0 0. 5 24 1. 5 74. 1 0. 5 00 0. 5 74. 1 O. 5 00 1.0 73. 6 0. 5 60 l. 5 73. 4 1. O 24 0. 5 78. 6 l. 0 24 1.0 78. 1 1.0 24 2.0 78. 1 1.0 60 0. 5 78. 6 1. 0 60 l. 0 78. 6 1.0 60 2.0 77. 4 2.0 24 0. 5 78. 3 2.0 24 1.0 78. 4 2.0 24 2.0 78. l 2. 0 00 0. 5 77.8 2. 0 00 1. 0 77. 3 2. 0 00 2. 0 76. 7

The above results show that best performance is obtained for any given alkali concentration under fairly narrow time and temperature conditions and that at 2% alkali the higher temperature and longer times gave somewhat less favorable results. The adverse elfect of longer times and higher temperatures becomes still more pronounced at still higher alkali concentration.

EXAMPLE 4 A sample of an 88% yield mixed Ohio Hardwood sulfite pulp (40-47% red oak, 20-23% white oak, 10% maple and 20%30% miscellaneous species including ash, polar, willow, beech and elm) which had been sulfite cooked at a pH of 6.8 prior to refining, and which had a brightness of 39.2% was acid-treated as described in Example 1. Portions thereof were then treated 1 hour at 43 C.

and 15% consistency with 0.5, 1.0 and 3.5% caustic soda. Following washing, the alkali-treated samples, and one which had been acid-treated but not alkali-treated, were bleached with peroxide as generally described in Example 1 (h) under the time and temperature conditions indicated below. The results were as follows:

Alkali Bleaching Percent Sample Treatment, Brightness Percent Temp, C. Time, hrs. Alter NaOtI Bleaching 1 Acid-treated only.

It will be noted that with this particular pulp, the use of only 0.5% caustic soda in the alkali treatment gave the best result.

EXAMPLE 5 Three different samples, A, B, C, of mixed Ohio Hardwood pulps of the same composition as that of Example 4, but which had been subjected to sulfite cooks at pH 6.8 prior to fiberization in the refiner to pulp yields of 88, 83 and 79%, respectively, were washed thoroughly with water. Each sample was divided into two portions. The first portion of each was treated for 1 hour at consistency and 24 C. with 1% caustic soda, washed thoroughly, then bleached with perioxide. The second portion of each was not pretreated with alkali but instead was bleached directly. The bleaching with peroxide was carried out in all instances as described in Example 1 (b). The results were as follows:

Percent Brightness Percent brightness Alter The above data show that for these pulps, the increases in brightness attributable to the alkali pretreatment ranged from 3 to 5 brightness points. It is noteworthy that when pulp A was bleached with 1% sodium hydrosulfite as described in Example 1, the brightness increased to only 47.6% which is a poor response compared to the brightness gains normally achieved when commercial ground- Woods are bleached with hydrosulfitc under the same conditions.

The invention involves subjecting the high yield hardwood sulfite pulp to an alkali treatment followed by washing prior to bleaching it with an alkaline peroxide bleach solution. If the original pulp contains sufficient amounts of heavy metal compound impurities which would catalyze decomposition of the peroxide in the bleaching stage, acidification of the pulp with a strong mineral acid such as sulfuric, hydrochloric, or phosphoric acid, followed by washing to remove such impurities prior to the bleaching stage is desirable. Such acidification will preferably precede the alkali treatment. If the original pulp is reasonably free of such impurities, the acidifying treatment can be omitted. In any case, the acid treatment is not essential to the practice of the invention since the alakli pretreatment will result in a substantial increase in brightness of the peroxide-bleached pulp whether or not the pulp is pretreated with an acid.

The embodiments of the invention in which an exclusive property or privilege is claimed are as follows:

1. The method of bleaching a high-yield hardwood sulfite pulp comprising treating said pulp at a pulp consistency of 5 to 40% with 0.3 to 3% of an alkali, calculated as caustic soda and based upon the dry weight of the pulp, at a temperature not exceeding 70 C. for a time not exceeding 3 hours, washing the pulp, and then bleaching the pulp with an aqueous alkaline hydrogen peroxide bleach solution.

2. The method of claim 1 wherein the alkali treatment of the pulp is eifected at 20 to 45 C. for a time of 0.5 to 1.5 hours employing 0.5 to 1.5% causting Soda, based upon the dry weight of the pulp.

3. The method of bleaching a high-yield hardwood sulfite pulp comprising treating said pulp at a pulp consistency of 5 to 40% with 0.3 to 2% caustic soda, based upon the dry weight of said pulp, at a temperature of 20 to C. for a time of 0.1 to 2 hours, washing said pulp, then bleaching said pulp with an alkaline solution of hydrogen peroxide containing 0.1 to 2.5% H 0 and 3 to 7.5% sodium silicate, all based upon the dry Weight of pulp, said solution of hydrogen peroxide having a total alkalinity calculated as NaOH, of from 0.5 to 4% based upon the dry weight of the pulp.

4. The method of claim 3 wherein the alkali treatment is efiected at 20 to 45 C. for a time of 0.5 to 1.5 hours employing 0.5 to 1.5% caustic soda, based upon the dry weight of the pulp.

5. The method of claim 1 employing a high-yield poplar sulfite pulp.

6. The method of claim 3 employing a high-yield poplar sulfite pulp.

References Cited UNITED STATES PATENTS 2,435,566 2/1948 Adams 162-78 X 2,860,944 11/1958 Young 16278 X 2,872,280 2/1959 Kindron 162-78 DONALL H. SYLVESTER, Primary Examiner.

H. CAINE, Examiner. 

1. THE METHOD OF BLEACHING A HIGH-YIELD HARDWOOD SULFITE PULP COMPRISING TREATING SAID PULP AT A PULP CONSISTENCY OF 5 TO 40% WITH 0.3 TO 3% OF AN ALKALI, CALCULATED AS CASUTIC SODA AND BASED UPON THE DRY WEIGHT OF THE PULP, AT A TEMPERATURE NOT EXCEEDING 70*C. FOR A TIME NOT EXCEEDING 3 HOURS, WASHING THE PULP, AND THEN BLEACHING THE PULP WITH AN AQUEOUS ALKALINE HYDROGEN PEROXIDE BLEACH SOLUTION. 